The Energy Audit-Air Conditioners

Summer has arrived, bringing warm temperatures and higher humidity.  As a kid, I remember hot nights sleeping in the upper story of an old farm house, often also suffering from sunburn.  Now, I can’t imagine not having at least one cool room in the house for relief.  What a wimp I’ve become.  Many homes in Minnesota have some sort of air conditioning today.  This blog posting is going to discuss the different types, efficiencies, and operating costs of common air conditioners along with the basics of how they operate.

A little history. The first air conditioner was invented by Willis Carrier in 1902 to solve a problem in an industrial application.  Interestingly, the first residential air conditioner was installed in 1913 in a mansion in Minneapolis of all places.  We’ve come a long way in the past 100 years.  Air conditioners are installed in nearly 90% of all new homes in the United States today.

Let’s get started with some definitions:

Central air conditioning.  Air conditioning distributed through a home’s forced air duct system.  The system consists of an outdoor condensing unit and the indoor evaporator coil, which is typically located above or below the air handler of the forced air system.

Window air conditioning.   A single, self-contained air conditioner installed into an open window. Typically installed in the

Window air conditioner

spring and removed in the fall. This type of air conditioner will only work with certain window sizes and styles.

Wall air conditioning.  A self-contained air conditioner installed in an exterior wall, looks very similar to a window air conditioner.  Typically, permanently installed.

Portable air conditioner.  An air conditioner that is placed within a room and has one or two flexible ducts mounted in a window.   Commonly used when a window AC unit will not fit in a window because of size or window type.  Portable air conditioners typically have the highest operating cost.

Portable air conditioner

Mini-split air conditioner.  An air conditioner with a wall mounted air handling unit and an outdoor condenser.  Popular in homes with hot water heating systems.  The heat pump version (see heat pump definition) of this system is becoming a popular heating and cooling system, especially in high-performance houses.

Multi-split air conditioner.  An air conditioner with two or more wall mounted air handling units and a single outdoor condenser.  Also available as an air source heat pump.

Ground source heat pump.  A geothermal heating and cooling system that uses the earth to store energy.  This system very efficiently moves heat and usually has a low operating cost, but a very high install cost.

Air source heat pump.  A heating and cooling system similar to a ground source heat pump.  Instead of using the earth for heat storage, temperatures are moved from the air.  Basically, an air conditioner that can operate in reverse to heat a home.

Condenser.  The outside unit of an air conditioner or air source heat pump.  Heat is moved from inside the home to the condenser and outside.

A-Coil.  The inside portion of an air conditioner or heat pump, typically installed above or below a furnace or air handler’s ductwork.  Commonly called an evaporator coil.

SEER – Seasonal energy efficiency ratio.  The performance rating of an air conditioner or air source heat pump.  The higher the number, the more efficient the unit.  A window AC unit might have a SEER rating of 10, with the most efficient air source heat pump rated at over 30.

How an air conditioner works.  According to Wikipedia:

In the refrigeration cycle, heat is transported from a colder location to a hotter area.  As heat would naturally flow in the opposite direction, work is required to achieve this.  A refrigerator is an example of such a system, as it transports the heat out of the interior and into its environment.  Refrigerant is used as the medium which absorbs and removes heat from the space to be cooled and subsequently ejects that heat elsewhere.

The process is based on the laws of thermodynamics, which I wrote a basic blog about here.   Back to Wikipedia:

Circulating refrigerant vapor enters the compressor, where its pressure and temperature are increased.  The hot, compressed refrigerant vapor is now at a temperature and pressure at which it can be condensed and is routed through a condenser.  Here it is cooled by air flowing across the condenser coils and condensed into a liquid.  Thus, the circulating refrigerant removes heat from the system and the heat is carried away by the air.  The removal of this heat can be greatly augmented by pouring water over the condenser coils, making it much cooler when it hits the expansion valve.
The condensed, pressurized, and still usually somewhat hot liquid refrigerant is next routed through an expansion valve (often nothing more than a pinhole in the system’s copper tubing) where it undergoes an abrupt reduction in pressure.  That pressure reduction results in flash evaporation of a part of the liquid refrigerant, greatly lowering its temperature.  The cold refrigerant is then routed through the evaporator.  A fan blows the interior warm air (which is to be cooled) across the evaporator, causing the liquid part of the cold refrigerant mixture to evaporate as well, further lowering the temperature. The warm air is therefore cooled and is pumped by an exhaust fan/ blower into the room.  To complete the refrigeration cycle, the refrigerant vapor is routed back into the compressor.  In order for the process to have any efficiency, the cooling/evaporative portion of the system must be separated by some kind of physical barrier from the heating/condensing portion, and each portion must have its own fan to circulate its own “kind” of air (either the hot air or the cool air).

This process is the same whether it is the air conditioning in your car, your home, or your refrigerator in your kitchen or chest freezer in your basement. Another benefit of air conditioning is when the warm interior air is moved across the cold evaporator, the moisture in the air condenses on the evaporator coil, removing the humidity from the air.  Properly sized air conditioners (ones that do not short cycle) also work as dehumidifiers, another blog I wrote that you can read here.  This is the reason for the drain hose on central air conditioning or heat pump systems or water dripping outside off a window air conditioner.

Now that we know the basics of how an air conditioner works, lets look at the efficiency rating or SEER of an air conditioner.  A higher SEER rating means a more efficient and lower operating cost unit.  There is a simple formula used to calculate SEER.  The formula is: British Thermal Unit (BTU) / Watts per hour. We will use a window unit as an example.  According to the nameplate rating, the air conditioner is rated at 12,100 BTU’s and consumes 1058 watts per hour.  The SEER rating of the unit is 11.43 SEER.  Far less efficient than the 30+ SEER rating of an efficient air source heat pump, but typical for lower cost air conditioners.  Let’s compare the window unit to a portable air conditioner.  A name plate on the portable AC unit states it cools at 6,500 BTU’s and uses 1152 watts per hour.

Outdoor condenser

The SEER rating is 5.6 SEER.  The examples above are actual air conditioner models with information gathered from their name plates.  The portable unit is much less efficient.  The SEER rating was not printed on the packaging of the unit, I see why.

Which type of AC to purchase.  The most efficient model is the ground source heat pump but is by far the most expense heating and cooling system. Costs of this system can easily be $50,000 for a closed loop system.  My advice to someone thinking about installing a ground source heat pump is to install an air source heat pump and a back-up system.  The money saved could be invested into energy saving measures in the home, such as added insulation, air sealing, or upgraded windows which will further lower heating and cooling costs.

An air source heat pump is the next most efficient model.  These systems are available in central air, mini-split, multi-split, and upper end (meaning higher cost) wall and window units.  The air source heat pump will cost more than the standard air conditioner, but with the added benefit of supplying heating and the lower operating cost, the increase in purchase price will eventually pay for itself.  Some new mini-split units have the ability to heat to temperatures below -20 degrees and are commonly used in high-performance houses.

Last is the standard air conditioner.  Available in all the types listed in the definition section, my suggestion is to purchase the most efficient, highest SEER model you can afford.  Energy Star rated models are a good starting point.  Be sure the unit is sized correctly for your home, this is true whether installing a ground source, air source, or standard air conditioner.  Often, one air conditioner is replaced by another of equal size.  The unit could be over sized, which will short cycle.  A short cycling air conditioner or heat pump will not properly cool or remove humidity within the home, and possibly prematurely fail due to the constant on and off cycling.

Cost of operation. There is a fairly simple way to calculate how much an air conditioner or heat pump is costing to operate.  You will need some information from the name plate of the unit, along with an estimate of hourly run time and the cost of power in your area.  The formula is wattage used by the product divided by 1000.  This gives us the kilowatt usage per hour. (Power is billed by the kilowatt).  Kilowatt (kWh) x percentage of time per hour the unit operates x the cost of power.  We will use the portable air conditioner from above for this example.  The unit consumes 1152 watts / 1000 = 1.152 kWh x .25 operating time (we will assume the unit operates 15 minutes every hour) x $.10 = $.0288 per hour.  Take the .0288 x 24 hours per day x 30 days per month and the cost is $20.73 per month.  Some nameplates will not have the wattage of the unit.  They will include amperage and voltage.  Multiplying the two together will equal the wattage. 120 volts x 9.6 amps = 1152 watts.

Lowering your operating cost. The easiest way to lower your air conditioning cost is to not operate the air conditioning unit until you need to.  Close windows, drapes and blinds during the day.  Open windows at night.  Using ceiling fans and box fans to move air can make it feel cooler.  Another option may be available.  Some electricity providers offer cycled air conditioning programs.  These programs cycle your air conditioner in 15-minute intervals in exchange for reduced electricity rates.  The current reduced rate for a local electricity provider is $0.0615 per kWh, or roughly half the rate normally charged.

There’s a lot of information in this posting, and a lot more technical data that I did not include.  Feel free to ask questions about air conditioning, or any other blog posting I have published.  Thanks to Wikipedia for the explanation of how an air conditioner works.  See you next week.

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